Follow-Up to #48: Add Paste Stack state tests

[alvst]

Summary

Adds Pesty’s first SwiftPM test target and focused Paste Stack state coverage.

• Verifies that deleting the active saved stack promotes the next deck.
• Verifies pasted entries move behind pending entries and keep selection correct.
• Verifies history-retention cleanup removes empty decks and promotes the remaining stack.
• Verifies saved-stack Codable round-tripping preserves entry state.

PasteSequence now has an internal initializer so tests can run with isolated state instead of mutating the application singleton.

Dependency

Builds on #48 because it covers saved-deck deletion behavior.

Validation

• swift test — 4 tests passed
• git diff --check